The invention relates to an exposure control circuit for a camera of TTL reflective photometry type, and more particularly, to an exposure control circuit for a camera of so-called TTL direct photometry type in which light from an object being photographed and passing through a taking lens is caused to be reflected by the surface of a shutter blind or a film to be incident upon a photoelectric transducer element which is used for purpose of photometry, allowing a resulting photocurrent to be immediately integrated to provide an output which is utilized for exposure control.
A single lens reflex camera of TTL direct photometry type is illustrated in FIG. 1. Specifically, there is shown a movable mirror 1 in its up position. Arranged in opposing relationship with a first shutter blind 2 and a film 3, which are shown in overlapping relationship with each other, is a photoelectric transducer element 4, which is used for purpose of photometry. The transducer element 4 is disposed on a mounting substrate 9. The movable mirror 1 is movable from its position 1A, shown in phantom line, to its up position shown as a shutter release is operated. When in its phantom line position, the mirror reflects light from an object being photographed and passing through a taking lens 8 to a focussing glass 5, and thence through pentaprism 6 and eyepiece 7 for observation by a viewer. After the movable mirror has moved to its up position, the light passing through the lens 8 is allowed to be incident upon the first shutter blind 2, which reflects the light to redirect it to the transducer element 4 for photometry. Subsequently, as the blind 2 runs, the film 3 becomes exposed to reflect the light to the transducer element 4, again for purpose of photometry. In this manner, the transducer element 4 produces a photometric output, which is used to provide an exposure control.
As is well recognized, with a focal plane shutter of the blind type, the film surface is initially covered by a first blind which comprises a black cloth. As the first blind moves in response to a shutter release operation, the film surface becomes exposed, and after a proper exposure period, a second shutter blind, which is also formed by a black cloth, runs to cover the film surface again. Consequently, a large difference between the optical reflectivity of the shutter blind surface and film surface causes a large change to be produced in the amount of light which the transducer element 4 receives. This means that a direct integration of a photocurrent produced by the transducer element 4 and which is proportional to the amount received by it cannot provide an accurate exposure period.
To cope with this problem, a conventional exposure control of the type described is designed to provide a reflectivity of a blind surface which is substantially equal to that of a film surface so that the transducer element 4 produces a photocurrent of a constant magnitude independently of the position of the shutter blind which it assumes during its running. A specific technique to provide an equal reflectivity for the blind surface comprises printing a patterned material on the blind surface which exhibits the same reflectivity as the film surface. However, the front surface of the shutter blind is usually formed by a cloth and its rear surface by a rubber lined cloth, and hence it is very difficult to treat the surface as by printing, resulting in a very expensive arrangement. In addition, variations in the pattern being printed cause a change in the reflectivity. Furthermore, winding the shutter blind at a high speed degrades the planarity of the blind and may cause an exfoliation of the pattern printed. Finally, it will be appreciated that the interior of a camera is usually provided with a black delustering paint in order to reduce stray light within a mirror box and to prevent a leakage of light to the film and the occurrence of ghosts and flares. Printing a reflective pattern on the blind surface diminishes these light extinction effects, giving rise to the occurrence of ghosts and flares.
To overcome this difficulty, the use of an equal reflectivity for the blind surface and the film surface is avoided in the prior art by providing an exposure control circuit which compensates for any exposure error which results from differential reflectivities. However, proposed techniques resulted in a complex circuit arrangement. A logarithmic compression technique may also be employed, but involves a response lag caused by a logarithmic compression diode. In particular when taking a picture in darkness (an exposure over a prolonged period of time), the accuracy of exposure control is degraded. This problem remains unsolved in a prior art exposure control circuit, disclosed in U.S. Pat. No. 4,072,961, in which a correction for the photometric value corresponding to the reflection from the first blind surface is made when the majority of the film surface becomes exposed.